Thermostable chimeric nucleic acid polymerases and uses thereof

ABSTRACT

Novel thermostable chimeric nucleic acid polymerases and methods for their generation and use are disclosed. It is shown that these chimeric nucleic acid polymerases, such as DNA polymerases, can be constructed using enzymatically active domains, isolated from different proteins or chemically synthesized. It is demonstrated that chimeric nucleic acid polymerases of the present invention possess the chemical and physical properties of their component domains (e.g., exonuclease activity, thermostability) and that the chimeric polymerases are thermostable.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 10/216,682, filed Aug. 8, 2002 (currently pending), which is a continuation of International Application No. PCT/EP01/01790, filed Feb. 16, 2001 (now abandoned), which is a continuation-in-part of U.S. Ser. No. 09/506,153, filed Feb. 17, 2000 (now abandoned), the disclosures of which are incorporated herein by reference in their entirety.

SEQUENCE LISTING

This application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated by reference in its entirety. Said ASCII copy, created Dec. 16, 2013, is named 0051_(—)0003US2—Sequence_Listing.txt and is 84489 bytes in size.

FIELD OF THE INVENTION

The present invention is in the field of molecular biology. The present invention is directed to novel thermostable chimeric enzymes useful for the generation of nucleic acids, methods for making thermostable chimeric nucleic acid polymerases, and methods useful for polymerizing nucleic acids using a thermostable chimeric nucleic acid polymerase. Specifically, the invention is directed to chimeric thermostable DNA polymerases and their uses.

BACKGROUND OF THE INVENTION

Nucleic acid polymerases are an important class of compounds that enzymatically link (polymerize) nucleotides to form larger polynucleotide chains (e.g., DNA or RNA strands). Nucleic acid polymerases typically utilize a template polynucleotide (in either a single-strand or double-strand form) for nucleic acid synthesis, as in conventional nucleic acid replication, transcription, or reverse transcription. Other nucleic acid polymerases, e.g., terminal transferase (TdT), are capable of de novo polymerization, that is, template independent nucleic acid synthesis.

All known nucleic acid polymerases possess an enzymatic domain that catalyzes the formation of a phosphodiester bond between two nucleotides, utilizing the 5′ carbon triphosphate of one nucleotide and the 3′ carbon hydroxyl group of another nucleotide. Nucleic acid polymerases synthesize nascent polynucleotides by linking the 5′ phosphate of one nucleotide to the 3′ OH group of the growing polynucleotide strand. This process is known and commonly referred to by persons skilled in the art as 5′-3′ polymerization.

In addition, nucleic acid polymerases possess a wide range of ancillary chemical properties useful for nucleic acid synthesis. These properties include, but are not limited to:

-   -   product and/or template specificity (e.g., RNA or DNA);     -   single-strand or double-strand template specificity;     -   processivity—a measure of the ability of a nucleic acid         polymerase to generate a nascent polynucleotide from a template         polynucleotide without dissociating from the template;     -   extension rate—a measure of the rate at which nucleotides are         added to a growing polynucleotide strand;     -   fidelity—a measure of the accuracy (or conversely the error         rate) with which a nucleic acid polymerase synthesizes a         polynucleotide complementary to a template polynucleotide;     -   nick translation—the ability of a nucleic acid polymerase to         degrade the preceding nucleotide strand of a double strand         molecule simultaneous to polymerizing a nascent strand;     -   proofreading—the ability of a nucleic acid polymerase to remove         an incorrectly linked nucleotide from a polynucleotide before         further polymerization occurs; and     -   thermostability—the ability of a nucleic acid polymerase to         retain activity after exposure to elevated temperatures.

Many of these properties are the result of one or more discrete functional domains within a polymerase holoenzyme. Three extensively studied enzymatically active domains of nucleic acid polymerase include: a 5′-3′ polymerase domain, responsible for polynucleotide synthesis; a 5′-3′ exonuclease domain, responsible for polynucleotide digestion of the 5′ end of a polynucleotide, useful for nick translation; and a 3′-5′ exonuclease domain, responsible for polynucleotide digestion of the 3′ end of a polynucleotide, allowing for proofreading, and thus improving the fidelity of the polymerase. Some studies indicate that selection, incorporation, and extension of the correct nucleotide, versus an incorrect nucleotide, is a variable property of the 5′-3′ polymerase domain, thus affecting polymerase fidelity in concert with proofreading activity (Mendelman et al., 1990; Petruska et al., 1988).

DNA polymerases can be categorized into six families based on amino acid homology. These families consist of pol I, pol α, SONDZEICHEN pol β, SONDZEICHEN DNA-dependent RNA polymerase, (Joyce and Steitz, 1994). Table 1 summarizes the enzymatic features of a few representative DNA polymerases.

TABLE 1 DNA polymerase enzymatic activity (N terminus --------- C terminus) 5′-3′ 3′-5′ 5′-3′ de novo DNA exonu- exonu- poly- Thermo- poly- polymerase clease clease merase stability merase E. coli pol I (+) (+) (+) (−) (−) Klenow fragment (−) (+) (+) (−) (−) E. coli pol II (−) (+) (+) (−) (−) E. coli pol III (+) (+) (+) (−) (−) T4 pol (−) (+) (+) (−) (−) T7 pol (−) (+) (+) (−) (−) M-MuLV RT (−) (−) (+) (−) (−) TdT (−) (−) (+) (−) (+) Taq pol (+) (−) (+) (+) (−) Stoffel fragment (−) (−) (+) (+) (−) Tbr pol (+) (−) (+) (+) (−) Tli pol (−) (+) (+) (+) (−) Tma pol (−) (+) (+) (+) (−) Tth pol (+) (−) (+) (+) (−) Pfu pol (−) (+) (+) (+) (−) Psp pol (−) (+) (+) (+) (−) Pwo pol (−) (+) (+) (+) (−)

Because of the diversity of properties and characteristics potentially exhibited by nucleic acid polymerases generally, practitioners in the art have sought to modify, to alter, or to recombine various features of nucleic acid polymerases in an effort to develop new and useful variants of the enzyme. Initially, polymerase truncations and deletions were developed. The Klenow fragment, for example, was the first nucleic acid polymerase variant developed. Klenow fragments exist as a large C-terminal truncation of DNA polymerase I (pol I), possessing an enzymatically active 3′-5′ exonuclease and 5′-3′ polymerase domains, but lacking altogether the 5′-3′ exonuclease domain of native pol I (Klenow and Henningsen, 1970; Jacobson et al., 1974; and Joyce and Grindley, 1983).

Since the advent of the polymerase chain reaction (PCR) methodology (including derivative methodologies such as reverse transcription PCR, or RT-PCR), resilient nucleic acid polymerases, capable of withstanding temperature spikes as high as 95° C. without a subsequent significant loss in enzymatic activity (i.e., thermostable) have become vital tools in modern molecular biology. The use of thermostable enzymes to amplify nucleic acid sequences is described in U.S. Pat. Nos. 4,683,195 and 4,683,202. A thermostable DNA polymerase from Thermus aquaticus (Taq) has been cloned, expressed and purified from recombinant cells (Lawyer et al., 1989; U.S. Pat. Nos. 4,889,818 and 5,079,352. PCR is also described in many U.S. patents, including U.S. Pat. Nos. 4,965,188, 4,683,195, 4,683,202, 4,800,159, 4,965,188, 4,889,818, 5,075,216, 5,079,352, 5,104,792, 5,023,171, 5,091,310, and 5,066,584.).

As depicted in Table I, Taq DNA polymerase possesses enzymatically active 5′-3′ polymerase and 5′-3′ exonuclease domains, but it exhibits only background levels of 3′-5′ exonuclease activity (Lawyer et al., 1989; Bernard et al., 1989; Longley et al., 1990). Crystallographic data revealed that Taq polymerase contains a 3′-5′ exonuclease domain (Eom et al., 1996); comparisons of the crystal structure of the Klenow fragment from Bacillus DNA polymerase I, Taq DNA polymerase, and E. coli DNA polymerase I have shown, however, that critical residues required to carry out a 3′-5′ exonuclease activity are missing in the 3′-5′ exonuclease domain of Taq DNA polymerase (Kiefer et al., 1997). Park et al. (1997), have determined that Taq DNA polymerase possesses none of three sequence motifs (Exo I, II, and III) within the 3′-5′ exonuclease domain and necessary for 3′-5′ exonuclease activity. Because Taq polymerase exhibits essentially no 3′-5′ exonuclease activity (i.e., proofreading capability), the error rate of Taq DNA polymerase is high compared to other DNA polymerases that possess an enzymatically active 3′-5′ exonuclease domain (Flaman et al., 1994). The Taq DNA polymerase structure thus comprises a 5′-3′ exonuclease domain occurring at the N-terminal region of the polypeptide (residues 1-291), followed by an enzymatically inactive 3′-5′ exonuclease domain (residues 292-423), and a C-terminal 5′-3′ polymerase domain (Park et al., 1997).

Since Taq DNA polymerase does not possess an enzymatically active 3′-5′ exonuclease domain, providing a proofreading feature to the polymerase, the use of Taq DNA polymerase becomes less desirable for most nucleic acid amplification applications, e.g., for PCR sequencing protocols or amplification for protein expression, which require complete identity of replication products to the template nucleic acid. Depending on the phase of PCR during which an error becomes incorporated into the PCR product (e.g., in an early replication cycle), the entire population of amplified DNA could contain one or more sequence errors, giving rise to a nonfunctional and/or mutant gene product. Nucleic acid polymerases that possess an enzymatically active 3′-5′ exonuclease domain (i.e., proofreading activity), therefore, are especially preferred for replication procedures requiring high fidelity.

Due to the scientific and commercial importance of PCR in modern molecular biology, the reliance of PCR protocols on nucleic acid polymerases of particular characteristics, and in view of the enzymatic deficiencies of Taq polymerase, an enormous amount of research and development has focussed on developing new and useful thermostable DNA polymerase variants and/or assemblages.

One approach has been directed to the discovery and isolation of new thermophilic nucleic acid polymerases, which may possess a unique and/or improved collection of catalytic properties. As a result, thermostable nucleic acid polymerases have been isolated from a variety of biological sources, including, but not limited to, species of the taxonomic genera, Thermus, Thermococcus, Thermotoga, Pyrococcus, and Sulfolobus. These polymerases possess a variety of chemical characteristics, as illustrated in Table 1. Some of these naturally occurring thermostable DNA polymerases possess enzymatically active 3′-5′ exonuclease domains, providing a natural proofreading capability and, thus, exhibiting higher fidelity than Taq DNA polymerase. Naturally occurring proofreading thermostable polymerases include: Pfu polymerase (isolated from Pyrococcus furiosus), Pwo polymerase (isolated from Pyrococcus woesei), Tli polymerase (isolated from Thermococcus litoralis), and Psp polymerase (isolated from Pyrococcus sp. GB-D). All of these naturally occurring thermostable polymerases are commercially available (Tli polymerase and Psp polymerase are marketed as Vent® and Deep Vent SONDZEICHEN® DNA polymerase, respectively, by New England Biolabs, Beverly, Mass.). These DNA polymerases show slower DNA extension rates and an overall lower processivity when compared to Taq DNA polymerase, however, thus rendering these naturally occurring thermostable DNA polymerases less desirable for PCR, despite their higher fidelity.

In an effort to compensate for the deficiencies of individual thermostable polymerases, a second approach has been to develop multiple enzyme assemblages, combining, for example, Taq polymerase and a proofreading enzyme, such as Pfu polymerase or Vent SONDZEICHEN® polymerase. These multiple-enzyme mixtures exhibit higher PCR efficiency and reduced error rates when compared to Taq polymerase alone (Barnes, 1994). Mixtures of multiple thermostable enzymes are commercially available (e.g., the Failsafe™ PCR system from Epicentre, Madison, Wis.). PCR protocols utilizing multiple polymerase mixtures are still prone to error, however, and require the practitioner to perform preliminary experimental trials, to determine special optimized solution conditions necessary for multiple-enzyme reaction mixtures.

A third approach has been to develop new and useful variants of Taq polymerase through deletion/truncation techniques. The Stoffel fragment, for example, is a 544 amino acid C-terminal truncation of Taq DNA polymerase, possessing an enzymatically active 5′-3′ polymerase domain but lacking 3′-5′ exonuclease and 5′-3′ exonuclease activity. Other commercially available thermostable polymerase deletions include Vent SONDZEICHEN® (exo⁻) and Deep Vent SONDZEICHEN® (exo⁻) (New England Biolabs, Beverly, Mass.). Deletion mutations serve only to remove functional domains of a nucleic acid polymerase, however, and do not add any novel features or enzymatic properties.

Polymerase mutagenesis is yet another approach that has been attempted to develop new and useful nucleic acid polymerase variants. Park et al. (1997) performed site-directed mutagenesis of 4 amino acids in the enzymatically inactive 3′-5′ exonuclease domain of Taq polymerase in an effort to activate the proofreading ability of this domain. The resultant mutant exhibited an increase of exonuclease activity over that of naturally occurring Taq polymerase. The reported increase was a mere two-fold increase above background exonuclease activity, however; an insignificant rise in exonuclease activity that is unlikely to increase PCR fidelity.

Bedford et al. (1997) developed a recombinant mesophilic DNA pol I from E. coli. They succeeded to insert a thioredoxin binding domain from T7 DNA polymerase into E. coli pol I. The inserted 76 amino acid binding domain improved polymerase binding to a template polynucleotide, thus increasing the processivity of the recombinant E. coli pol I but did not improve or provide any novel enzymatic activity to the polymerase.

Recently Gelfand et al. (1999) combined fusion protein technology with mutagenesis to eliminate or substantially reduce 5′-3′ exonuclease activity and 3′-5′ exonuclease activity in recombinant polymerases. Once again, no improved or additional enzymatic activity was provided by the fusion polymerase.

Frey et al. (1999) attempted to engineer chimeric polymerases utilizing enzymatically active domains from Taq, Tne, and E. coli DNA polymerases. Although they successfully substituted the non-functional 3′-5′ exonuclease domain of Taq DNA polymerase with a functional 3′-5′ exonuclease domain from another DNA polymerase, their resultant chimeric polymerase lost significant, if not all, enzymatic activity after only one minute at 80° C. or 95° C. (i.e., they are not thermostable), and thus are not useful for performing PCR protocols without the successive addition of fresh polymerase for each cycle.

Despite these intense research efforts, there remains a need in the art for thermostable nucleic acid polymerases that possess improved or novel assemblages of enzymatically active domains. Despite its enzymatic deficiencies, Taq DNA polymerase remains the most widely used enzyme for processing in vitro amplification of nucleic acids. In particular, there has been long felt need for a nucleic acid polymerase possessing the 5′-3′ polymerization qualities of Taq polymerase, but which also possesses 3′-5′ exonuclease (proofreading) activity.

SUMMARY OF THE INVENTION

In response to the long felt need for new and useful nucleic acid polymerases, a novel approach for producing thermostable nucleic acid polymerases was invented. The present invention represents the first thermostable chimeric nucleic acid polymerase, useful for continuous PCR protocols, obtained by combining at least two enzymatically active domains from different proteins by means of recombinant DNA techniques.

The present invention is directed to novel thermostable chimeric enzymes useful for the generation of nucleic acids, methods for making thermostable chimeric nucleic acid polymerases, and methods useful for polymerizing nucleic acids using a thermostable chimeric nucleic acid polymerase. The thermostable chimeric nucleic acid polymerase of the present invention comprises at least two enzymatically active domains, which are non-naturally associated. The recombinant association of the enzymatically active domains results in a composite enzyme not found in nature. The thermostable chimeric nucleic acid polymerase of the present invention possesses new or improved catalytic properties compared to nucleic acid polymerases known in the art.

The thermostable chimeric nucleic acid polymerase of the present invention offers several advantages over previous approaches to develop novel nucleic acid polymerases. The present invention provides a single enzyme that possesses a suite of chemical properties, the combination of which may not necessarily exist in nature, but nonetheless is useful in molecular biology. The chimeric nucleic acid polymerase of the present invention eliminates the need to specifically develop multiple-enzyme reaction mixtures, which are often difficult to optimize and expensive to use, and the necessity to add successive amounts of fresh enzyme during each cycle of a PCR program. The invention thus facilitates the rapid, efficient, and accurate generation of nucleic acid molecules, particularly in regard to PCR protocols.

DEFINITIONS

As used herein, an “enzymatically active domain” refers to any polypeptide, naturally occurring or synthetically produced, capable of mediating, facilitating, or otherwise regulating a chemical reaction, without, itself, being permanently modified, altered, or destroyed. Binding sites (or domains), in which a polypeptide does not catalyze a chemical reaction, but merely forms noncovalent bonds with another molecule, are not enzymatically active domains as defined herein. In addition, catalytically active domains, in which the protein possessing the catalytic domain is modified, altered, or destroyed, are not enzymatically active domains as defined herein. Enzymatically active domains, therefore, are distinguishable from other (nonenzymatic) catalytic domains known in the art (e.g., detectable tags, signal peptides, alosteric domains, etc.).

As defined herein, a 3′-5′ exonuclease domain refers to any polypeptide capable of enzymatically cleaving a nucleotide from the 3′ end of a di- or polynucleotide, a 5′-3′ exonuclease domain refers to any polypeptide capable of enzymatically cleaving a nucleotide from the 5′ end of a di- or polynucleotide, and a 5′-3′ polymerase domain refers to any polypeptide capable of enzymatically linking the 5′ phosphate of one nucleotide to the 3′ OH group of another nucleotide.

Polypeptide domains that are “non-naturally associated”, refer to specific polypeptides that are not naturally produced within a single polypeptide; that is, the polypeptide domains are not naturally translated from a common nucleic acid transcript in a naturally occurring organism. Non-naturally associated polypeptide domains include domains isolated from functionally distinct proteins, separately produced by an organism of one or more species, or synthetically generated, as well as polypeptide domains isolated from functionally similar proteins, but naturally produced by organisms of different species, or synthetically generated. The term “non-naturally associated polypeptide domains” refers to domains that are associated or fused only through human intervention; the term expressly excludes naturally occurring enzymes or fragments thereof.

As used herein, the term “chimeric protein” encompasses all proteins that contain two or more polypeptide domains that are non-naturally associated (regardless of whether the domains are naturally produced by organisms of the same species, different species, or synthetically generated). A chimeric nucleic acid polymerase of the present invention must necessarily possess two or more non-naturally associated domains, as defined herein.

The term “thermostable” generally refers to the resilience of a substance to relatively high temperature treatment. A thermostable enzyme is an enzyme that retains its definitive enzymatic activity despite exposure to relatively high temperature. A thermostable nucleic acid polymerase, as generally understood by practitioners in the art and as defined herein, refers to a polymerase that is useful for PCR protocols; i.e., not requiring successive or supplemental addition of enzyme after each high temperature step of the PCR program cycle. The chimeric nucleic acid polymerase of the present invention is thermostable, in that it is useful for PCR protocols, because it does not require successive or supplemental addition of polymerase after each high temperature step of the PCR program cycle.

A preferred thermostable chimeric polymerase of the present invention is one that allows a thermal polymerase chain reaction to proceed with only an initial supply of polymerase at the start of the PCR program. Preferably, a thermostable chimeric nucleic acid polymerase retains some measurable enzymatic activity at its normal operating temperature (typically about 72° C.) after exposure to 95° C. for three minutes. More preferably, a thermostable chimeric nucleic acid polymerase is able to withstand one minute at 95° C. without significant loss (>5% loss) in enzymatic activity. In other words, a preferred thermostable chimeric nucleic acid polymerase retains at least about 95% of its polymerase activity at its normal operating temperature (typically about 72° C.) after one minute at 95° C. Even more preferably, a thermostable chimeric nucleic acid polymerase is able to withstand three minutes at 95° C. without significant loss in enzymatic activity. A most preferred thermostable chimeric nucleic acid polymerase is able to withstand ten minutes at 90° C. and still retain at least about 50% of its enzymatic activity at its normal operating temperature. In other words, the polymerase displays a “half life” (the length of time it takes for a substance to lose one half of its initial activity) of ten minutes at 90° C. Ideally, a thermostable chimeric nucleic acid polymerase displays a half-life comparable to the half-life measurement of naturally occurring thermostable nucleic acid polymerases. For example a most desirable thermostable chimeric nucleic acid polymerase displays a half-life at 90° C. comparable to that of Taq polymerase, approximately 90 minutes.

The present invention is directed generally to all thermostable chimeric nucleic acid polymerases comprising at least two non-naturally associated enzymatically active domains. As defined herein, a nucleic acid polymerase is any enzyme that catalyzes the formation of chemical bonds between (chemically bonds) nucleotides to form polynucleotide chains, that is, any enzyme that promotes nucleic acid polymerization. The thermostable chimeric nucleic acid polymerases of the present invention include all types of nucleic acid polymerases, without limitation to product or template specificity, molecular requirements, or chemical properties (e.g., RNA vs. DNA, single strand vs. double strand, high fidelity, etc.).

One embodiment of the present invention is directed to a thermostable chimeric DNA polymerase, preferably a chimeric DNA polymerase wherein the enzymatically active domains are isolated from naturally occurring proteins from two or more species, or any mutants, variants, or derivatives thereof.

As used herein, mutant, variant, and derivative polypeptides refer to all chemical permutations of a given polypeptide, which may exist or be produced, that still retain the characteristic molecular activity that is definitive of that polypeptide.

The thermostable chimeric nucleic acid polymerase of the present invention is unexpected in view of the fact that enzymatically active domains may be isolated from a wide variety of sources, yet still retain their enzymatic activities (e.g., polymerase, exonuclease) and chemical properties (e.g., thermostability, processivity). Enzymatically active domains isolated from organisms of different taxonomic kingdoms and from completely different families of proteins may be fused to produce an entirely novel, yet functional, nucleic acid polymerase. For example, enzymatically active domains from a eubacterium polymerase of e.g., Taq polymerase may be chimerically joined with enzymatically active domains from an archaeon polymerase (e.g., Pwo, Sso, and Pho polymerases).

Retention of thermal stability in a fusion protein engineered from different thermophilic proteins is highly unexpected. Attempts to construct chimeric polymerases have failed to produce thermostable chimeric polymerases (see Frey et al., 1999). The underlying principles of thermal stability of proteins derived from thermophilic organisms are not known. Even small changes in the amino acid sequence of thermoresistant proteins result in a significant decrease in thermal stability and an associated reduction in enzymatic activity of the protein. Maintenance of, or an increase in, thermal stability of thermostable DNA polymerase has only been accomplished by truncation of a DNA polymerase (e.g., Barnes, 1995). The present invention represents the first chimeric nucleic acid polymerase, containing enzymatically active domains from different thermostable proteins, that possess thermostable properties.

In a preferred embodiment, at least one of the enzymatically active domains of the chimeric nucleic acid polymerase is isolated from a DNA polymerase produced by a thermophilic organism, preferably an organism of a genus selected from the group of genera consisting of: Thermus, Thermococcus, Thermotoga, Pyrococcus, Pyrodictium, Bacillus, Sulfolobus, and Methanobacterium. Most preferably, at least one of the enzymatically active domains of the chimeric nucleic acid polymerase is isolated from a DNA polymerase selected from the group consisting of: Thermoplasma acidophilum (Tac) polymerase; Thermus aquaticus (Taq) polymerase; Thermococcus barossii (Tba) polymerase; Thermus brockianus (Tbr) polymerase; Tfi polymerase; Thermus flavus (Tfl) polymerase; Thermococcus litoralis (Tli) polymerase; Thermus ruber (Tru) polymerase; Thermus thermophilus (Tth) polymerase; Pyrodictium abyssi (Pab) polymerase; Pyrococcus furiosus (Pfu) polymerase; Pyrococcus hellenicus (Phe) polymerase; Pyrococcus horikoshii (Pho) polymerase; Pyrococcus kodakarensis (Pko) polymerase; Pyrococcus sp. strain KOD1 (KOD) polymerase; Pyrococcus sp. strain ES4 (ES4) polymerase; Pyrodictium occultum (Poc) polymerase; Pyrococcus sp. GB-D (Psp) polymerase; Pyrococcus woesei (Pwo) polymerase; Thermotoga maritima (Tma) polymerase; Thermotoga neapolitana (Tne) polymerase; Bacillus sterothermophilus (Bst) polymerase; Sulfolobus acidocaldarius (Sac) polymerase; Sulfolobus solfataricus (Sso) polymerase; Methanobacterium thermoautotrophicum (Mth) polymerase; and mutants, variants, and derivatives thereof.

In another embodiment of the invention, the enzymatically active domains are selected from the group consisting of: 5′-3′ exonuclease domain, 3′-5′ exonuclease domain, and 5′-3′ polymerase domain. Preferably the enzymatically active domains are naturally occurring domains, isolated from two or more species, most preferably the enzymatically active domains are isolated from naturally occurring thermostable proteins, mutants, variants, or derivatives thereof.

Another aspect of the present invention relates to an isolated polynucleotide encoding a thermostable chimeric nucleic acid polymerase comprising at least two non-naturally associated enzymatically active domains. Preferably the enzymatically active domains are isolated from different species.

A related aspect of the invention is directed to a method for synthesizing a recombinant nucleic acid that encodes a thermostable chimeric nucleic acid polymerase comprising at least two non-naturally associated enzymatically active domains.

A further aspect of the invention relates to a vector comprising a polynucleotide that encodes a thermostable chimeric nucleic acid polymerase having at least two non-naturally associated enzymatically active domains. Preferred vectors are expression vectors, which will be suitable for production of the encoded chimeric nucleic acid polymerase in transformed host cells.

Another aspect of the invention includes a recombinant host cell transformed with a vector comprising a polynucleotide that encodes a thermostable chimeric nucleic acid polymerase possessing at least two non-naturally associated enzymatically active domains.

A related aspect of the invention is directed to a method for producing a thermostable chimeric nucleic acid polymerase comprising at least two non-naturally associated enzymatically active domains.

Another aspect of the invention is directed to a process of nucleic acid polymerization, which necessarily utilizes a thermostable chimeric nucleic acid polymerase having at least two non-naturally associated enzymatically active domains.

A related aspect of the invention is directed to a kit useful for polymerization of nucleic acid, comprising a thermostable chimeric nucleic acid polymerase having at least two non-naturally associated enzymatically active domains. Preferably, the kit further comprises at least one reagent suitable for nucleic acid polymerization. Most preferably, the kit further comprises at least one reagent selected from the group consisting of one or more additional enzymes, one or more oligonucleotide primers, a nucleic acid template, any one or more nucleotide bases, an appropriate buffering agent, a salt, or other additives useful in nucleic acid polymerization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of an ethidium bromide (EtdBr)-stained agarose gel, which depicts the polymerase activity of thermostable chimeric DNA polymerases using a primer extension reaction. Lane 1 shows a nucleic acid ladder, used as a gel reference marker. Lanes 2, 6 and 10 show negative controls (without addition of polymerase). Lane 3, 4, 5 show the activity of 0.05, 0.03 and 0.01 units Taq DNA polymerase, respectively. Lanes 7-9 illustrate polymerase activity of undiluted cleared lysate, a 1:1, and 1:5 diluted cleared lysate, of a Pho/Taq chimeric polymerase, respectively.

FIG. 2 is a photograph of an ethidium bromide (EtdBr)-stained agarose gel, which depicts the thermostability of a thermostable chimeric DNA polymerase compared to Taq DNA polymerase, using a primer extension reaction. DNA polymerases were incubated for various time spans at 90° C. and assayed for remaining polymerase activity. Lanes 1 and 11 show a nucleic acid ladder, used as a gel reference marker. Lanes 2, 10, 12, and 20 represent negative control reactions (without addition of polymerase). Lanes 3-9 and lanes 13-19 illustrate DNA polymerase activity after incubation of Taq DNA polymerase and a Pho/Taq chimeric DNA polymerase at 90° C. for 0, 10, 15, 30, 60, 90, and 120 min, respectively.

FIG. 3 is a photograph of an ethidium bromide (EtdBr)-stained agarose gel, which depicts 3′-5′ exonuclease activity of three different thermostable DNA polymerases. (A) illustrates PCR product using a wild type primer combination. (B) illustrates PCR product using a mutant primer pair. Lane 1 is a nucleic acid ladder, used as a gel reference marker. The PCR amplification product of Taq DNA polymerase is shown in lanes 2-5; Pfu DNA polymerase I PCR product is shown in lanes 6-9; and a Pho/Taq thermostable chimeric DNA polymerase PCR product is shown in lanes 10-13. Duplicate side-by-side reactions are shown representing undigested (the first and third lane for each enzyme used), and digested (the second and fourth lane for each enzyme used) PCR product.

FIG. 4 is a photograph of an ethidium bromide (EtdBr)-stained agarose gel, which illustrates the combined effect of primer extension efficiency and polymerase processivity on PCR efficiency of three different thermostable DNA polymerases. The photograph illustrates PCR products obtained in duplicate reactions using different primer extension times. (A) indicates PCR products obtained with Taq DNA polymerase. (B) illustrates PCR products obtained with a Pho/Taq thermostable chimeric DNA polymerase. (C) shows PCR products generated with Pfu DNA polymerase I. Lane 1 is a nucleic acid ladder, used as a gel reference marker. Lanes 2-3 show PCR products amplified after primer extension for 1 min. Lanes 4-5 show PCR products amplified after primer extension for 30 sec. Lanes 6-7 show PCR products amplified after primer extension for 10 sec. Lanes 8-9 show PCR products amplified after primer extension for 5 sec.

DETAILED DESCRIPTION OF THE INVENTION

Genetic engineering techniques were successfully employed to generate the first thermostable chimeric nucleic acid polymerase, containing enzymatically active domains, not naturally found within a single protein. The chimeric nucleic acid polymerase and methods described herein encompass all thermostable nucleic acid polymerases, without limitation to product or template specificity, molecular requirements, or chemical properties. For example, chimeric nucleic acid polymerases of the present invention include single or double strand DNA polymerases, RNA polymerases, and reverse transcriptases. Thermostable chimeric nucleic acid polymerases of the present invention may possess any number and/or combination of properties and features including, but not limited to, template dependence or independence, high processivity, high fidelity, proofreading, nick translation, and high extension rates. Persons skilled in the art will understand and appreciate that these features are due, in large part, to the presence and characteristics of discrete polypeptide domains within the holoenzyme. Essential to the chimeric nucleic acid polymerase of the present invention is that it possess at least two enzymatically active domains that are not naturally associated, and the chimeric nucleic acid is thermostable.

Enzymatically active domains may be isolated from any natural polypeptide, or may be synthetically produced. Natural polypeptides include any polypeptide found in nature, and from any organism of any taxonomic group. Enzymatically active domains useful in the present invention also include variant, mutant, or derivative forms of domains found in nature. Enzymatically active domains further include domains that may not be found in nature, e.g., polypeptides randomly generated or engineered in the laboratory or selected from a non-naturally generated library of polypeptides. For the purposes of this invention, enzymatically active domains need only necessarily possess an enzymatic activity that is functional within the chimeric nucleic acid polymerase of the invention. The thermostable chimeric nucleic acid polymerases of the present invention specifically contemplates incorporation into a nucleic acid polymerase, enzymatically active domains that are absent, inactive, or weakly active in the naturally occurring protein.

Persons skilled in the art will know and appreciate that a wide variety of enzymatic domains exist that perform the same or similar enzymatic functions. For example, DNA polymerases possess 3′-5′ exonuclease domains of a wide range of enzymatic functionality; from little or no 3′-5′ exonuclease activity (as seen in Taq polymerase), to fully functional 3′-5′ exonuclease activity (as seen in E. coli pol I), to thermostable 3′-5′ exonuclease activity (as seen in Pwo polymerase). It is understood by practitioners in the art that enzymatically active domains of individual polymerases are considered separate and distinct enzymatically active domains, as defined herein. Thus, the incorporation of an enzymatically active domain from one polymerase into a second polymerase produces, by definition, a chimeric polymerase, regardless of whether the second polymerase naturally possesses its own enzymatically active domain of similar functionality.

Preferably, genetic engineering techniques may be used to generate novel thermostable DNA polymerases possessing either 5′-3′ polymerase activity and 3′-5′ exonuclease activity; or 5′-3′ polymerase activity, 3′-5′ exonuclease activity and 5′-3′ exonuclease activity derived from different thermostable DNA polymerases, e.g. Taq polymerase, Pho polymerase, Pwo polymerase, and Sso polymerase.

Preferred thermostable chimeric nucleic acid polymerases of the present invention include a 5′-3′ polymerase domain of Taq polymerase. For example, the Stoffel fragment is a 544 residue N-terminal deletion of Taq polymerase possessing an enzymatically active 5′-3′ polymerase domain and an enzymatically inactive 3′-5′ exonuclease domain. Generally, a Taq 5′-3′ polymerase domain is at least about 544 residues in length, and includes any mutant, variant, or derivative of the Stoffel fragment of Taq polymerase, as defined herein. A 552 amino acid polypeptide, residue numbers 281-832 of Taq polymerase (SEQ ID NO:1), is an especially preferred enzymatically active Taq 5′-3′ polymerase domain useful in the present invention.

Alternatively, the thermostable chimeric nucleic acid polymerases of the present invention may include a 5′-3′ polymerase domain of Tth polymerase. Tth polymerase is capable of reverse transcription. Thermostable chimeric nucleic acid polymerases, which include the Tth 5′-3′ polymerase domain, therefore, may be used for reverse transcription reactions (e.g., RT-PCR). Preferably, the 5′-3′ polymerase domain of Tth polymerase is about 562 residues in length, including residue numbers 273-834 of Tth polymerase (SEQ ID NO:2), and includes any mutant, variant, or derivative thereof.

Preferred thermostable chimeric nucleic acid polymerases of the present invention also include an enzymatically active 3′-5′ exonuclease domain of a thermostable polymerase. Preferred 3′-5′ exonuclease domains include the enzymatically active 3′-5′ exonuclease domains of Pho polymerase, Pwo polymerase, and Sso polymerase. Most preferred are residues 1-396 of Pho polymerase (SEQ ID NO:3), residues 1-396 of Pwo polymerase (SEQ ID NO:4), residues 1-421 of Pwo polymerase (SEQ ID NO:5), residues 1-508 of Sso polymerase (SEQ ID NO:6), and any mutants, variants, or derivatives of any one of these 3′-5′ exonuclease domains, as defined herein.

A process for synthesizing a recombinant nucleic acid encoding a thermostable chimeric nucleic acid polymerase of the invention necessarily comprises isolating at least two nucleic acid fragments each encoding at least one enzymatically active domain, which is not naturally associated with the other enzymatically active domain (i.e., derived from separate polypeptides), and genetically combining the nucleic acids of the enzymatically active domains to form a chimeric nucleic acid.

For production of thermostable chimeric nucleic acid polymerases according to the invention, the nucleic acid encoding a chimeric nucleic acid polymerase may be stably inserted into a genetic vector, preferably the nucleic acid is operably inserted into an expression vector, and most preferably the vector construct is capable of replication within a host organism, such that the nucleic acid encoding a thermostable chimeric nucleic acid polymerase is capable of being transcribed and translated into a polypeptide. A preferred mode of making the chimeric nucleic acid polymerase of the present invention includes culturing a host cell containing a nucleic acid encoding a thermostable chimeric nucleic acid polymerase under conditions suitable for expression of the chimeric nucleic acid polymerase by the host cell, and isolating the chimeric nucleic acid polymerase expressed from said cell culture.

Methods for generating recombinant nucleic acids, vector construction, host cell transformation, and polypeptide expression systems useful in the practice of this invention can involve a wide variety of modern genetic engineering techniques, tools, and biological sources that are well known in the art and routinely practiced by those skilled in the art. Exemplary techniques and methods are described in detail herein by way of preferred example, but are not limiting to the practice of the invention. The present invention incorporates by reference in their entirety techniques and supplies well known in the field of molecular biology, including, but not limited to, techniques and supplies described in the following publications:

-   Ausubel, F. M. et al. eds., Short Protocols In Molecular Biology     (4th Ed. 1999) John Wiley & Sons, NY. (ISBN 0-471-32938-X). -   Freshney, R. I. Culture of Animal Cells (1987) Alan R. Liss, Inc. -   Old, R. W. & S. B. Primrose, Principles of Gene Manipulation: An     Introduction To Genetic Engineering (3d Ed. 1985) Blackwell     Scientific Publications, Boston. Studies in Microbiology; V.2:409     pp. (ISBN 0-632-01318-4). -   Sambrook, J. et al. eds., Molecular Cloning: A Laboratory Manual (2d     Ed. 1989) Cold Spring Harbor Laboratory Press, NY. Vols. 1-3. (ISBN     0-87969-309-6). -   Winnacker, E. L. From Genes To Clones: Introduction To Gene     Technology (1987) VCH Publishers, NY (translated by Horst     Ibelgaufts). 634 pp. (ISBN 0-89573-614-4).

The thermostable chimeric nucleic acid polymerases described herein are especially useful for generating a desired target nucleic acid. Thermostable chimeric nucleic acid polymerases of the invention, having at least two enzymatically active domains that are not naturally associated may be utilized under conditions sufficient to allow polymerization of a nascent nucleic acid. Generally, this method includes any method of nucleic acid generation, replication, amplification, transcription, or reverse transcription known in the art that utilizes a conventional nucleic acid polymerase, wherein the nucleic acid polymerase is substituted or combined with a chimeric nucleic acid polymerase of the present invention. Preferably the method of amplification is polymerase chain reaction, utilizing a thermostable chimeric nucleic acid polymerase. PCR is described herein as an exemplary protocol capable of utilizing the compositions and methods of the present invention without limitation. Persons skilled in the art will understand that the present invention has utility in other processes requiring the polymerization of nucleic acid (e.g., RT-PCR).

PCR is a technique well known in the art. PCR is used to amplify nucleic acids by subjecting a reaction mixture to cycles of: (i) nucleic acid denaturation, (ii) oligonucleotide primer annealization, and (iii) nucleic acid polymerization. Preferred reaction conditions for amplification comprise thermocycling, i.e., alternating the temperature of the reaction mixture to facilitate each of the steps of the PCR cycle. PCR is typically extended through multiple cycles of denaturation, annealization and replication, augmented (optionally and preferably) with an initial prolonged denaturation step and a final prolonged extension (polymerization) step. To perform the repetitive steps of thermocycling, it is preferable to employ an enzyme that is capable of tolerating exposure to relatively high temperature without a subsequent significant loss in enzyme activity; i.e., a thermostable enzyme. The use of a thermostable enzyme for PCR protocols permits the repetitive steps of increasing and decreasing reaction temperatures without the need to supplement, or otherwise add, enzyme after each successive high temperature step of the PCR program cycle.

Also included in the invention is a kit that includes a thermostable chimeric nucleic acid polymerase and one or more additional reagents suitable for nucleic acid polymerization reactions. Such components may include, but are not limited to: one or more additional enzymes, one or more oligonucleotide primers, a nucleic acid template, any one or more nucleotide bases, an appropriate buffering agent, a salt, or other additives useful in nucleic acid polymerization.

Additional enzymes of the kit include any enzyme that may be used in combination with the thermostable chimeric nucleic acid polymerase of the invention. For example, multiple-polymerase kits are known in the art. Numerous polymerases are known and commercially available to persons skilled in the art, and include DNA polymerases, RNA polymerases, and reverse transcriptases (commercial suppliers include: Roche Diagnostics., Indianapolis, Ind.; Life Technologies, Inc., Rockville, Md.; New England Biolabs, Inc., Beverly, Mass.; Perkin Elmer Corp., Norwalk, Conn.; Pharmacia LKB Biotechnology, Inc., Piscataway, N.J.; Qiagen, Inc., Valencia, Calif.; Stratagene, La Jolla, Calif.).

Oligonucleotide primers useful in the present invention may be any oligonucleotide of two or more nucleotides in length. Preferably, PCR primers are about 15 to about 30 bases in length and are not palindromic (self-complementary) or complementary to other primers that may be used in the reaction mixture. Primers may be, but are not limited to, random primers, homopolymers, or primers specific to a target oligonucleotide template (e.g., a sequence specific primer). Oligonucleotide primers are oligonucleotides used to hybridize to a region of a target nucleic acid to facilitate the polymerization of a complementary nucleic acid. In PCR protocols, primers serve to facilitate polymerization of a first nucleic acid molecule complementary to a portion of an oligonucleotide template, and also to facilitate replication of the oligonucleotide. Any primer may be synthesized by a practitioner of ordinary skill in the art or may be ordered and purchased from any of a number of commercial venders (e.g., from Roche Diagnostics, Indianapolis, Ind.; Life Technologies, Inc., Rockville, Md.; New England Biolabs, Inc., Beverly, Mass.; Pharmacia LKB Biotechnology, Inc., Piscataway, N.J.). It will be understood that a vast array of primers may be useful in the present invention, including those not specifically disclosed herein, without departing from the scope or preferred embodiments thereof.

A nucleic acid template is defined as any polynucleotide molecule used to provide a nucleic acid sequence from which a polynucleotide complementary to the template may be generated. The synthesis of DNA from a DNA template may be accomplished according to the invention by utilizing a thermostable chimeric DNA polymerase. The synthesis of RNA from a DNA template may be accomplished according to the invention by utilizing a thermostable chimeric RNA polymerase. The synthesis of DNA from an RNA template may be accomplished according to the invention by utilizing a thermostable chimeric nucleic acid polymerase that exhibits reverse transcriptase activity.

Nucleotide bases useful in the present invention may be any nucleotide useful in the polymerization of a nucleic acid. Nucleotides may be naturally occurring, unusual, modified, derivative, or artificial. Nucleotides may be unlabeled, or detectably labeled by methods known in the art (e.g., using radioisotopes, vitamins, fluorescent or chemiluminescent moieties, digoxigenin). Preferably the nucleotides are deoxynucleoside triphosphates, dNTPs (e.g., dATP, dCTP, dGTP, dTTP, dITP, dUTP, α-thioSONDZEICHEN-dNTPs, biotin-dUTP, fluorescein-dUTP, digoxigenin-dUTP, 7-deaza-dGTP). dNTPs are also well known in the art and are commercially available (e.g., from Roche Diagnostics, Indianapolis, Ind.; New England Biolabs, Inc., Beverly, Mass.; Pharmacia LKB Biotechnology, Inc., Piscataway, N.J.).

Buffering agents and salts useful in the present invention provide appropriate stable pH and ionic conditions for nucleic acid synthesis. A wide variety of buffers and salt solutions and modified buffers are known in the art that may be useful in the present invention, including agents not specifically disclosed herein. Preferred buffering agents include, but are not limited to, TRIS, TRICINE, BIS-TRICINE, HEPES, MOPS, TES, TAPS, PIPES, CAPS. Preferred salt solutions include, but are not limited to solutions of; potassium chloride, potassium acetate, potassium sulfate, ammonium sulfate, ammonium chloride, ammonium acetate, magnesium chloride, magnesium acetate, magnesium sulfate, manganese acetate, sodium chloride, sodium acetate, lithium chloride, and lithium acetate.

Other additives capable of facilitating nucleic acid generation and amplification, other than those disclosed for the first time by this invention, are known in the art. In accordance with the compositions and methods of this invention, one or more of these additives may be incorporated in a DNA/RNA polymerization kit according to the present invention to optimize the generation and replication of polynucleotides. Additives may be organic or inorganic compounds. Agents useful in the present invention include, but are not limited to, polypeptides such as phosphatase, human serum albumin, bovine serum albumin (BSA), ovalbumin, albumax, casein, gelatin, collagen, globulin, lysozyme, transferrin, α-lactalbumin, β-lactoglobulin, phosphorylase b, myosin, actin, β-galactosidase, lectins, E. coli single-stranded binding (SSB) protein, phage T4 gene 32 protein, and the like, or fragments or derivatives thereof. Examples of nonpolypeptide additives include, but are not limited to; homopolymeric nucleic acid, heteropolymeric nucleic acid, tRNA, rRNA, sulfur-containing compounds, acetate-containing compounds, dimethylsulfoxide (DMSO), glycerol, formamide, betain, tetramethylammonium chloride (TMAC), polyethylene glycol (PEG), Tween 20, NP 40, ectoine, and polyoles.

It will be readily apparent to those skilled in the art that other suitable modifications and adaptations of the compositions and methods of the invention described herein are obvious and may be made without departing from the scope of the invention or the embodiments disclosed herein. Having now described the present invention in detail, the same will be more clearly understood by reference to the following examples, which are included for purposes of illustration only and are not intended to be limiting of the invention.

Example 1 Construction of a Thermostable Chimeric DNA Polymerase Gene

Chimeric thermostable DNA polymerase constructs containing enzymatically active domains from different (source) thermostable DNA polymerases were generated using recombinant DNA techniques. The 3′-5′ exonuclease domain of various thermostable polymerases were recombinantly linked to the 5′-3′ polymerase domain of Taq polymerase or Tth polymerase. The particularly preferred enzymatic domains and domain borders, described herein in detail, were selected and tested as preferred embodiments, and are not to be considered limiting in scope of the thermostable chimeric nucleic acid polymerase of the invention, or the enzymatically active domains useful therein.

Appropriate microbial strains or genomic DNA preparations, from which the enzymatically active domains used in the construction of chimeric nucleic acid polymerase were isolated, were purchased from commercial suppliers, e.g., from DSMZ GmbH (Deutsche Sammlung von Mikroorganismen and Zellkulturen GmbH), Braunschweig, Germany. Specifically chosen strains included Thermus aquaticus (order # DSM 625), Thermus thermophilus (order # DSM 579), Pyrococcus furiosus (order # DSM 3638). Pyrococcus woesei (order # DSM 3773). Pyrococcus horikoshii (DSM 3638), and Sulfolobus solfataricus (order # DSM 5833). A multiplicity of genomic DNA extraction, purification, and isolation techniques useful to obtain the desired enzymatically active domains are well known in the art.

Modified PCR amplification techniques and/or cloning procedures such as restriction digestion and ligation using appropriate enzymes were used to obtain the chimeric DNA polymerase constructs. Primers appropriate to amplify polynucleotides encoding particular enzymatic domains from the source thermostable DNA polymerases were synthesized according to the nucleotide sequences of the source thermostable DNA polymerase. DNA sequences of the source thermostable DNA polymerases are published in GenBank. The synthesis of oligonucleotide primers is well known to practitioners in the art, and may also be ordered from commercial oligonucleotide suppliers (e.g., Life Technologies, Gaithersburg, Md.).

PCR primers were of special design. The primers contained a nucleotide sequence complementary to the terminal region of a particular enzymatic domain of interest within a source DNA polymerase. The primers also contained a noncomplementary nucleotide sequence region as well to provide; i) an appropriate restriction enzyme site, to facilitate genetic manipulation (e.g., vector insertion), or ii) sequence information (e.g., complementarity), to facilitate fusion to a second, non-naturally associated enzymatic domain. For example, primers designed to facilitate fusion of a 3′-5′ exonuclease domain to a 5′-3′ polymerase domain contained a sequence, one half of which was complementary to a terminal region of the 3′-5′ exonuclease domain of interest (e.g., residues 388-396 of Pho polymerase) and one half of which was complementary to a terminal region of the 5′-3′ polymerase domain (e.g., residues 281-288 of Taq polymerase).

As an initial step, various enzymatic domains were amplified by PCR. The PCR reaction mixture contained: 2.5 units of Taq polymerase (Qiagen, Valencia, Calif.) and 0.1 to 0.2 units of Pfu polymerase (Stratagene, La Jolla, Calif.); an appropriate amount of the specially designed primers, as described above (0.2 to 1.0 μM); genomic DNA isolated from the appropriate microorganism containing the source thermostable polymerase; and 200 μM of each dNTP in a 1×PCR buffer (Qiagen, Valencia, Calif.). A 3-step PCR cycling program was run, consisting of an initial denaturation step at 94° C., an annealing step and an extension step. The PCR ran for 25-35 cycles, depending upon the desired amount of product. The size of the PCR product was checked by agarose gel electrophoresis against an appropriate DNA size marker. The correctly sized PCR product was gel-purified using the QIAquick™ Gel Extraction Kit (Qiagen, Valencia, Calif.).

Once isolation and amplification of the polynucleotides encoding the enzymatic domains chosen for chimeric polymerase construction were obtained, the component enzymatic domains were combined, in equivalent concentrations, in a composite PCR reaction, together with 2-5 units of Pfu polymerase (Stratagene, La Jolla, Calif.), and 200 μM of each dNTP in 1×PCR buffer (Qiagen, Valencia, Calif.). This PCR mixture did not contain any primer oligonucleotides. This reaction mixture was subjected to 10 to 15 PCR cycles.

During the composite PCR, the single strand polynucleotides encoding each of the enzymatically active domains hybridize at their respective terminal regions of complementarity (due to the specially designed primers as described above). The hybridized single strand polynucleotides encoding each of the enzymatically active domains form a single composite polynucleotide template, thus serving as primers for each other. Pfu polymerase extends the 3′ terminal end of each of the enzymatically active domains, creating a single polynucleotide containing the chimeric DNA polymerase gene construct.

After the initial 10 to 15 cycles of chimeric DNA polymerase gene construction, oligonucleotide primers, appropriate to amplify the full-length chimeric DNA polymerase gene, were added to the PCR mixture. The PCR ran for 20-30 additional cycles, depending upon the desired amount of chimeric DNA polymerase PCR product. The size of the PCR product was checked by agarose gel electrophoresis and the correctly sized PCR product was gel-purified as described above.

The purified chimeric DNA polymerase gene was then subjected to restriction digestion with the appropriate restriction enzyme to cut the polynucleotide at restriction sites located at the terminal ends of the chimeric DNA polymerase gene. These sites were originally generated by the specially designed primers described above.

Example 1.1 Construction of a Pho/Taq Thermostable Chimeric DNA Polymerase Gene

A polynucleotide encoding the enzymatically active 3′-5′ exonuclease domain of Pho DNA polymerase was linked to a polynucleotide encoding the enzymatically active 5′-3′ polymerase domain and the nonfunctional 3′-5′ exonuclease domain of Taq DNA polymerase. A polynucleotide encoding amino acids 271-832 (SEQ ID NO:7) of Taq DNA polymerase was recombinantly linked to the 3′ end of a polynucleotide encoding amino acids 1-396 (SEQ ID NO:3) of Pho DNA polymerase following the procedures detailed in Example 1 above, producing a polynucleotide that encodes a novel Pho/Taq thermostable chimeric DNA polymerase (SEQ ID NO:8).

Example 1.2 Construction of a Pwo/Taq Thermostable Chimeric DNA Polymerase Gene

A polynucleotide encoding the enzymatically active 3′-5′ exonuclease domain of Pwo DNA polymerase was linked to a polynucleotide encoding the enzymatically active 5′-3′ polymerase domain of Taq DNA polymerase. A polynucleotide encoding amino acids 271-832 (SEQ ID NO:7) of Taq DNA polymerase was recombinantly linked to the 3′ end of a polynucleotide encoding amino acids 1-396 (SEQ ID NO:4) of Pwo DNA polymerase following the procedures detailed in Example 1 above, producing a polynucleotide that encodes a novel Pwo/Taq thermostable chimeric DNA polymerase (SEQ ID NO:9).

Example 1.3 Construction of a Sso/Taq Thermostable Chimeric DNA Polymerase Gene

A polynucleotide encoding the enzymatically active 3′-5′ exonuclease domain of Sso DNA polymerase was linked to a polynucleotide encoding the enzymatically active 5′-3′ polymerase domain of Taq DNA polymerase. A polynucleotide encoding amino acids 281-832 (SEQ ID NO:1) of Taq DNA polymerase was recombinantly linked to the 3′ end of a polynucleotide encoding amino acids 1-508 (SEQ ID NO:6) of Sso DNA polymerase following the procedures detailed in Example 1 above, producing a polynucleotide that encodes a novel Sso/Taq thermostable chimeric DNA polymerase (SEQ ID NO:10).

This chimeric construct, possessing a smaller Taq 5′-3′ polymerase domain than that used in Examples 1.1 and 1.2, also demonstrates that specifically determined domain borders of an enzymatic domain are not essential to the invention. What is essential for the domain is that it retain its definitive enzymatic activity.

Example 1.4 Construction of Variant Thermostable Chimeric DNA Polymerase Genes

To further demonstrate that a thermostable chimeric nucleic acid polymerase may be generated using an enzymatically active domain of varying domain borders (provided the enzymatic activity of the domain is retained), a Pwo/Taq chimeric DNA polymerase variant of the thermostable chimeric polymerase generated in Example 1.2 was constructed. This variant construct comprised a polynucleotide encoding amino acids 271-832 (SEQ ID NO:7) of Taq DNA polymerase recombinantly linked to the 3′ end of a polynucleotide encoding amino acids 1-421 (SEQ ID NO:5) of Pwo DNA polymerase following the procedures detailed in Example 1 above, producing a polynucleotide that encodes a second novel Pwo/Taq thermostable chimeric DNA polymerase (SEQ ID NO:11).

Example 1.5 Construction of a Pho/Tth Thermostable Chimeric DNA Polymerase Gene

To demonstrate that a thermostable chimeric nucleic acid polymerase may be generated using an enzymatically active polymerase domain other than that of Taq polymerase, a polynucleotide encoding the enzymatically active 3′-5′ exonuclease domain of Pho DNA polymerase was linked to a polynucleotide encoding the enzymatically active 5′-3′ polymerase domain of Tth DNA polymerase. A polynucleotide encoding amino acids 273-834 (SEQ ID NO:2) of Tth DNA polymerase was recombinantly linked to the 3′ end of a polynucleotide encoding amino acids 1-396 (SEQ ID NO:3) of Pho DNA polymerase following the procedures detailed in Example 1 above, producing a polynucleotide that encodes a novel Pho/Tth thermostable chimeric DNA polymerase (SEQ ID NO:12).

This chimeric construct, possessing a Tth 5′-3′ polymerase domain that is also capable of reverse transcription activity, also demonstrates a thermostable chimeric nucleic acid polymerase of the present invention useful for RT-PCR protocols.

Example 1.6 Construction of a Thermostable Chimeric DNA Polymerase Gene Encoding More than Two Enzymatically Active Domains

The chimeric nucleic acid polymerase gene of the invention may encode two or more enzymatically active domains, of which two more domains are non-naturally occurring. In addition the enzymatically active domains may be derived from any polypeptide source naturally occurring or synthetically produced.

For example, the practitioner may wish to construct a thermostable chimeric nucleic acid polymerase possessing both the 5′-3′ polymerase domain and the 5′-3′ exonuclease domain of Taq polymerase, as well as the 3′-5′ exonuclease domain of another polymerase (e.g., Pho polymerase). In this instance, a polynucleotide encoding the 5′-3′ exonuclease domain of Taq polymerase (known to be contained within amino acids 1-291 of Taq polymerase) would be recombinantly linked to 5′ end of a polynucleotide encoding the 3′-5′ exonuclease domain of Pho polymerase (e.g., SEQ ID NO: 3) and the 5′-3′ polymerase domain of Taq DNA polymerase (e.g., SEQ ID NOs: 1 or 7), which was earlier demonstrated in Examples 1.1 and 1.4.

Example 2 Construction of a Thermostable Chimeric DNA Polymerase Vector

The isolated chimeric DNA polymerase genes of Examples 1.1 through 1.5 were each ligated into a vector, linearized using the appropriate restriction enzyme. Ligation was performed overnight at 16° C. using T4 DNA ligase and an appropriate buffer (Life Technologies, Gaithersburg, Md.) in a final volume of 20 μl.

Example 3 Construction of a Thermostable Chimeric DNA Polymerase Host Cell

The ligated recombinant vectors of Example 2 were used to transform calcium-competent M15[pRep4] cells (Qiagen, Valencia, Calif.) or DH5SONDZEICHENα competent cells. Aliquots of the transformation mixture were spread onto agar plates containing ampicillin and kanamycin (for M15[pRep4] cells), or ampicillin only (for DH5α competent cells), and incubated overnight at 37° C.

Colonies of successfully transformed cells were transferred to LB media containing the appropriate antibiotic selection, and incubated overnight. Plasmid isolation preparations were performed using QIAprep™ Spin Kit or Plasmid Midi Kit (both from Qiagen, Valencia, Calif.). Presence of the chimeric DNA polymerase gene was verified by restriction digest analysis and the chimeric DNA polymerase gene sequenced by techniques well known in the art.

The chimeric DNA polymerase genes were cloned into either pQE-30 or pQE-31 expression vectors (Qiagen, Valencia, Calif.) containing a six-histidine tag sequence preceding the respective DNA polymerase sequence.

Example 4 Expression and Purification of a Thermostable Chimeric DNA Polymerase

Thermostable chimeric DNA polymerase gene expression of the successfully transformed host cells from Example 3, was induced by IPTG. Harvested cells were lysed by sonification and lysozyme treatment or a simple heat treatment. Chimeric His-tagged protein was purified in batch format using Ni-NTA agarose (Qiagen, Valencia, Calif.) following standard protocol procedures.

Eluates were ultrafiltrated using NanosepSONDZEICHEN® ultrafiltration units (Pall Deutschland GmbH Holding, Dreieich, Germany). Alternatively, heat treated cleared lysate was centrifuged through Ultrafree filterunits 300,000 (Sigma, Deisenhofen, Germany), to remove contaminating nucleic acids, and was subsequently concentrated using NanosepSONDZEICHEN® or MicrosepSONDZEICHEN® ultrafiltration units (Pall Deutschland GmbH Holding, Dreieich, Germany).

Concentrated samples were mixed with a storage buffer containing 20 mM TrisHCl (pH 8.0 at 20° C.), 100 mM KCl, 1 mM EDTA, 0.5% (v/v) Nonidet P-40 substitute, 0.5% (v/v) Tween 20 and 50% (v/v) glycerol. Chimeric polymerase preparations were stored at −20° C. In some cases, the cleared lysate of the polymerase preparation was directly used for subsequent analysis; chimeric polymerase preparations were then stored at +4° C.

Example 5 5′-3′ Polymerase Activity of Thermostable Chimeric DNA Polymerases

To demonstrate the polymerase activity of thermostable chimeric DNA polymerases produced from Example 4, an assay for measuring primer extension activity was performed. This assay is based on the difference in mobility of single-versus double-strand DNA molecules on an agarose gel in the presence of a DNA intercalating dye. Annealing of a primer to a single-stranded DNA molecule creates a priming site for a DNA polymerase. The primer is then extended by the polymerase, converting the single-strand DNA into double-strand molecules. The extension rate is dependent upon the polymerase used. The final amount of DNA extension (i.e., polymerization) is dependent on the amount of polymerase provided, the extension rate of the polymerase, and the length of time the reaction is allowed to proceed.

All polymerization reaction mixtures contained 50 ng M13 mp 18 DNA (20 fmol; 7250 nt), 0.1 μM 30-mer oligonucleotide primer 5′-TTTCCCAGTCACGACGTTGTAAAACGACGG-3′ (SEQ ID NO: 13), and 50 μM of each dNTP in 10 μl of 10 mM Tris HCl.

Polymerization reactions containing Taq DNA polymerase and a thermostable chimeric DNA polymerase were performed in 1×PCR buffer (Qiagen, Valencia, Calif.).

Taq DNA polymerase was used for external standard reactions (0.05, 0.03, 0.01 units) in order to determine polymerase activity of the thermostable chimeric DNA polymerases. DNA polymerases were diluted in the reaction buffer containing 1 μg/ml bovine serum albumin (BSA) to compensate for possible protein interactions with the surface of the polypropylene tube.

The assay was performed in a MJ Research PTC-200 Thermocycler (Biozym, Hess. Oldendorf, Germany) or a Biometra UnoII Thermocycler (Biometra, Göttingen, Germany). The thermal program consisted of a 10 sec. denaturation step 94° C.; a 30 sec. annealing step at 55° C.; and a 3 min. polymerization step at 72° C. Heating of the reaction mixture to 94° C. was done to destroy possible secondary structures of the single-stranded M13 DNA and to facilitate specific primer annealing during the lowering of reaction temperature to 55° C.

Results of primer extension reactions at 72° C. were reproducible. After completing the reaction, reaction products were mixed with 1 μl gel loading solution (50% Glycerol, 1×TAE buffer, 0.02 mg/ml Bromphenol blue) and loaded on a 1% agarose gel containing 0.5 μg/ml ethidium bromide. The gel was run at 80 mA for 15 min in 1×TAE buffer. These conditions facilitated discrimination between extended-(ds) and non-extended (ss) M13 DNA fragments. The results, as represented in FIG. 1, illustrate the polymerase activity of the thermostable chimeric polymerase is comparable to that of wild type Taq polymerase.

Example 6 Thermostability of Chimeric DNA Polymerases

The primer extension assay described in Example 5 was also used to measure the resilience of chimeric DNA polymerases to thermal degradation (i.e., thermostability). Heat-treatment of chimeric DNA polymerases (0.2 units) consisted of incubation of the enzyme for 0, 10, 15, 30, 60, 90 and 120 min at 90° C., followed by primer extension at 72° C. Polymerase activity of heat-treated chimeric polymerase was compared to untreated chimeric DNA polymerase based on the amount of polymerized (i.e., double strand) M13 DNA. The same assay was performed, under identical reaction conditions, on identical amounts of Taq DNA polymerase, as a standard. A control consisted of a polymerase reaction mixture, without any DNA polymerase. After completing the reaction, reaction products were mixed with 1 μl gel loading solution (50% Glycerol, 1×TAE buffer, 0.02 mg/ml Bromphenol blue), and loaded on a 1% agarose gel containing 0.5 μg/ml ethidium bromide. The gel was run at 80 mA for 15 min in 1×TAE buffer. The results, presented in FIG. 2 and quantified in Table 2 below, are representative of the thermostability assay.

TABLE 2 Thermostability of chimeric polymerase compared to Taq polymerase Incubation Pho/Taq Chimeric Taq Polymerase at 90° C. (min.) Polymerase % Activity % Activity 0 100 100  10 84 99 15 84 89 30 82 74 60 66 69 90 53  31* 120 45 43 *single non-reproducible data; value expected to be higher

FIG. 2 and Table 2 confirm the thermostability of the chimeric polymerase of the present invention. Table 2 illustrates that although the activity of the chimeric DNA polymerase shows an initial drop in activity (within the first 10 min at 90° C.) greater than that of Taq DNA polymerase, the overall thermostability is comparable to Taq DNA polymerase. Chimeric DNA polymerase of the invention displays the same half life at 90° C. as Taq DNA polymerase (approximately 90 min).

The thermostability assay was also performed under extreme temperature conditions. The primer extension assay was run after heat-treatment at 95° C. for 0, 3, 5, and 10 min. The results, quantified in Table 3 below, are representative of the 95° C. thermostability assay, and further confirm that the chimeric DNA polymerase of the present invention is highly thermostable.

TABLE 3 Thermostability of chimeric polymerase Incubation Pho/Taq Chimeric at 95° C. (min.) Polymerase % Activity 0 100 3 100 5 86 10 86

These results confirm the thermostability of the chimeric DNA polymerase of the present invention, making it useful for in vitro reactions under heat denaturing conditions such as PCR, without requiring successive addition of enzyme at each cycle of the PCR program.

Example 7 3′-5′ Exonuclease Activity of Thermostable Chimeric DNA Polymerases

Fidelity of DNA replication is based on a two step process: misinsertion and misextension. In PCR, if the DNA polymerase inserts an incorrect nucleotide, and the resulting 3′-mismatched terminus of the growing DNA chain is not extended, the truncated primer extension product cannot be amplified during subsequent PCR cycles since the downstream primer binding site is missing. Additionally, mismatched termini are less efficiently extended than DNA ends harboring the complementary base. DNA polymerases possessing an enzymatically active 3′-5′ exonuclease domain are capable of removing a misincorporated nucleotide, thus increasing fidelity of the PCR product and increasing primer extension efficiency.

A PCR and restriction endonuclease digestion assay, developed to assess the ability of thermostable DNA polymerases to remove mismatched primer termini by 3′-5′ exonuclease activity, was performed using the protocol disclosed in U.S. Pat. No. 5,491,086 (incorporated by reference). Wild type primers, perfectly matching the BamHI restriction enzyme recognition sequence in the Taq polymerase gene, and mutant primers, possessing a 3′-mismatch (employing every possible combination) to the first nucleotide of the BamHI restriction enzyme recognition sequence, were used in side-by-side PCR trials.

Wild type primers to 5′-GCACCCCGCTTGGGCAGAG-3′ (SEQ ID NO:14) and 5′-TCCCGCCCCTCCTGGAAGAC-3′ (SEQ ID NO:15) yield a 151 bp PCR product that becomes digested upon incubation with BamHI restriction enzyme, generating a 132 bp and 19 bp fragment.

Three forward primers containing a single 3′-mismatched nucleotide representing a C:A, C:T, and C:C mismatch to SEQ ID NO:14 were used as mutant primers. Any extension product from these mutant primers would corrupt the BamHI restriction site, rendering the resulting PCR products unaffected by BamHI digestion, thus leaving the 151 bp PCR product intact. The presence of an enzymatically active 3′-5′ exonuclease domain, would correct the 3′-mismatched nucleotide of the mutant primer, however, thus restoring the BamHI restriction site, rendering the PCR product susceptible to BamHI digestion, thus producing the 132 bp and 19 bp digestion fragments.

Using this PCR fidelity assay, the chimeric thermostable DNA polymerase was tested for the ability to correct a 3′-primer mismatch during PCR. Chimeric polymerase trials were run in parallel with wild type Taq DNA polymerase and Pfu DNA polymerase I. The Taq DNA polymerase trials served as a negative control, representing a DNA polymerase possessing an enzymatically inactive 3′-5′ exonuclease domain (i.e., proofreading capability). The Pfu DNA polymerase I trials served as a positive control, representing a thermostable DNA polymerase possessing an enzymatically active 3′-5′ exonuclease domain.

PCR mixtures comprised 20 ng plasmid pQE-31 containing the (target) Taq polymerase gene sequence; 0.5 units of the test DNA polymerase; 0.4 μM of the appropriate trial primers (wild type vs. mutant primers); 200 μM of each dNTP; 1× Qiagen PCR buffer (Qiagen, Valencia, Calif.) or 1×Pfu reaction buffer (Stratagene, La Jolla, Calif.) and 1.5 mM MgCl₂ in a final reaction volume of 50 μl.

PCR was performed using a MJ Research PTC-200 Thermocycler (Biozym, Hess. Oldendorf, Germany) or a Biometra UnoII Thermocycler (Biometra, Göttingen, Germany). The PCR program consisted of an initial 1 min template denaturation step at 94° C. followed by 40 cycles of a 30 sec. denaturation step 94° C.; a 30 sec. annealing step at 62° C.; and a 1 min. polymerization step at 72° C. for 1 min. The PCR concluded with a final prolonged extension step for 2 min. at 72° C.

PCR products were analyzed on a 2% agarose gel by gel electrophoresis (approximately 35 min. at 85 volts) in 1×TAE electrophoresis buffer and Ethidium bromide. PCR products were visualized using UV irradiation, and quantified using the 200 bp DNA fragment of the Low DNA MassSONDZEICHEN™ Ladder (Life Technologies, Gaithersburg, Md., USA) as standard by gel densitometry. PCR products were purified using QIAquick™ PCR Purification Kit (Qiagen, Valencia, Calif.).

Identical amounts of PCR product were digested in the same final reaction volume using 1 unit BamHI (Life Technologies, Gaithersburg, Md., USA) per 100 ng PCR product and corresponding reaction buffer. Restriction digest was performed for 90 min. at 37° C. Digestion products were analyzed on a 4% MetaphorSONDZEICHEN® agarose gel (Biozym, Hess. Oldendorf, Germany) FIG. 3. is representative of the results of the 3′-5′ exonuclease activity assay.

FIG. 3(A) illustrates the PCR product of the three nucleic acid polymerases (Taq polymerase, Pfu polymerase, and the thermostable chimeric polymerase) using wild type primers. Alternating lanes represent undigested PCR product and PCR product subjected to BamHI digestion. Undigested product shows the intact 151 bp PCR product. Digestion treated product shows the 132 bp digestion fragment.

FIG. 3(B) illustrates the PCR product of the three polymerases (Taq polymerase, Pfu polymerase, and the thermostable chimeric polymerase) using mutant primers. Once again, alternating lanes represent undigested PCR product and PCR product subjected to BamHI digestion. Taq polymerase PCR product was unaffected by BamHI digestion (lanes 3 and 5), due to the lack of a BamHI site resulting for normal extension of the mutant primer. Pfu polymerase PCR product was effectively digested by BamHI (lanes 7 and 9), producing the expected 132 bp digestion fragment. These results are indicative of the proofreading ability (i.e., 3′-5′ exonuclease activity) of Pfu polymerase, which corrected the nucleotide mismatch of the mutant primer, thus restoring the BamHI site of the template DNA.

The thermostable chimeric polymerase PCR product displayed results similar to the Pfu polymerase PCR product. The chimeric polymerase PCR product was also effectively digested by BamHI (lanes 11 and 13), producing the expected 132 bp digestion fragment and indicative of polymerase proofreading ability. These results confirm that the thermostable chimeric polymerase, which possesses the 5′-3′ polymerase domain of Taq polymerase, also possesses an enzymatically active 3′-5′ exonuclease domain not naturally occurring in Taq polymerase.

Example 8 PCR efficiency of Thermostable Chimeric DNA Polymerases

PCR efficiency of a DNA polymerase can be described as the combined effect of primer extension activity and processivity of the enzyme. PCR efficiency of the thermostable chimeric DNA polymerase was tested in comparison with Taq DNA polymerase, known to possess a higher PCR efficiency than common proofreading polymerases, and Pfu DNA polymerase (both serving as controls).

One unit of the respective polymerase was used to amplify a 750 bp large product from human genomic DNA using a thermocycling profile with varying primer extension times at 72° C. Limiting primer extension time was used to measure polymerase efficiency in PCR, using the same amount of enzyme activity in the assay. Taq DNA polymerase was assayed in its optimized PCR buffer (Qiagen, Valencia, Calif.), a Pho/Taq thermostable chimeric DNA polymerase was used in a 1× buffer consisting of 50 mM TrisHCl (pH 8.9 at room temperature), 10 mM (NH₄)₂SO₄, and Pfu DNA polymerase was used in the reaction buffer supplied with the enzyme (Stratagene, La Jolla, Calif.). All reactions contained 1 unit of enzyme, 0.4 μM of each primer, 200 μM of each dNTP, and a final MgCl₂ concentration of 1.5 mM (Taq polymerase, chimeric DNA polymerase) or 2.0 mM (Pfu polymerase).

Thermocycling was performed in a Biometra Uno thermocycler using the following cycling conditions: initial denaturation at 94° C. for 3 min followed by a denaturation step at 94° C. for 30 sec, an annealing step at 60° C. for 30 sec, and a primer extension step at 72° C. for 1 min, 30 sec, 10 sec or 5 sec. The reaction proceeded for 34 cycles, and concluded with a final extension step at 72° C. for 10 min.

The results are depicted in FIG. 4. Taq DNA polymerase (A) shows a high PCR efficiency even when primer extension time is as low as 5 sec. The thermostable chimeric DNA polymerase (B) shows a higher PCR efficiency than Taq polymerase at extension times of 1 min and 30 sec, but a slightly lower efficiency than Taq polymerase at 5 sec extension time. Pfu DNA polymerase I (C) generates a visible PCR product only when using the 1 min extension time.

These results indicate that the overall processivity of the chimeric polymerase is comparable to that of Taq DNA polymerase, and is dramatically better than Pfu DNA polymerase I. The thermostable chimeric polymerase of the present invention performs as well as Taq DNA polymerase (the standard enzyme of PCR protocols), and outperforms Pfu DNA polymerase I (the standard enzyme for high fidelity PCR protocols). In addition, the thermostable chimeric polymerase of the present invention combines the beneficial features of each of the standard enzymes for PCR protocols formerly not obtained with either Taq DNA polymerase or proofreading polymerases: removal of misincorporated nucleotides required for high fidelity PCR, and high PCR efficiency.

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Each of the publications mentioned herein is incorporated by reference. 

We claim:
 1. A method of synthesizing a recombinant nucleic acid encoding a thermostable chimeric nucleic acid polymerase having two, non-naturally associated, enzymatically active domains, the method comprising: (a) isolating a first nucleic acid encoding a first enzymatically active domain; (b) isolating a second nucleic acid encoding a second enzymatically active domain, wherein said second enzymatically active domain is non-naturally associated with said first enzymatically active domain; and (c) combining said first nucleic acid and said second nucleic acid to form said recombinant nucleic acid encoding said thermostable chimeric nucleic acid polymerase.
 2. The method of claim 1, wherein said first enzymatically active domain is a 3′-5′ exonuclease domain.
 3. The method of claim 1, wherein said second enzymatically active domain is a 5′-3′ polymerase domain.
 4. The method of claim 1, wherein said isolating in (a) and (b) comprises amplifying the first nucleic acid and the second nucleic acid by polymerase chain reaction (PCR) with a PCR primer comprising a first nucleotide sequence complementary to a terminal region of a 3′-5′ exonuclease domain of said first nucleic acid and a second nucleotide sequence complementary to a terminal region of a 5′-3′ polymerase domain of said second nucleic acid.
 5. The method of claim 1, wherein said combining comprises hybridizing said first nucleic acid to said second nucleic acid to form a composite polynucleotide template, and amplifying said composite polynucleotide template to form said recombinant nucleic acid encoding said thermostable chimeric nucleic acid polymerase.
 6. The method of claim 2, wherein said 3′-5′ exonuclease domain comprises a 3′-5′ exonuclease domain of Pho DNA polymerase.
 7. The method of claim 6, wherein said 3′-5′ exonuclease domain comprises amino acid residues 1 to 396 of Pho DNA polymerase (SEQ ID NO:3).
 8. The method of claim 2, wherein said 3′-5′ exonuclease domain comprises a 3′-5′ exonuclease domain of Pwo DNA polymerase.
 9. The method of claim 8, wherein said 3′-5′ exonuclease domain comprises amino acid residues 1 to 396 of Pwo DNA polymerase (SEQ ID NO:4).
 10. The method of claim 8, wherein said 3′-5′ exonuclease domain comprises amino acid residues 1 to 421 of Pwo DNA polymerase (SEQ ID NO:5).
 11. The method of claim 2, wherein said 3′-5′ exonuclease domain comprises a 3′-5′ exonuclease domain of Sso DNA polymerase.
 12. The method of claim 11, wherein said 3′-5′ exonuclease domain comprises amino acid residues 1 to 508 of Sso DNA polymerase (SEQ ID NO:6).
 13. The method of claim 2, wherein said 3′-5′ exonuclease domain comprises a 3′-5′ exonuclease domain of Tpac DNA polymerase.
 14. The method of claim 13, wherein said 3′-5′ exonuclease domain comprises amino acid residues 1 to 395 of Tpac DNA polymerase (SEQ ID NO:16).
 15. The method of claim 3, wherein said 5′-3′ polymerase domain is a 5′-3′ polymerase domain of Taq DNA polymerase.
 16. The method of claim 3, wherein said 5′-3′ polymerase domain is a 5′-3′ polymerase domain of Tth DNA polymerase.
 17. The method of claim 15, wherein said 5′-3′ polymerase domain comprises amino acid residues 281 to 832 of Taq DNA polymerase (SEQ ID NO:1).
 18. The method of claim 15, wherein said 5′-3′ polymerase domain comprises amino acid residues 271 to 832 of Taq DNA polymerase (SEQ ID NO:7).
 19. The method of claim 16, wherein said 5′-3′ polymerase domain comprises amino acid residues 273 to 834 of Tth DNA polymerase (SEQ ID NO:2). 